1. Field of the Invention
The present invention relates to an image processing apparatus, a printing apparatus, and an image processing method. More particularly, the present invention relates to correction for suppressing an uneven density caused by variations of print characteristics of printing elements, the correction being performed in an image processing system for performing quantization with a dither matrix.
2. Description of the Related Art
The head shading (abbreviated as “HS”) technique disclosed in Japanese Patent Application Laid-open No. H10-13674(1998) has been known as one example of the correction for suppressing an uneven density of the aforementioned type. This HS technique is adapted to perform correcting based on information on ink ejection characteristics (i.e., print characteristics) of each of nozzles serving as printing elements. For example, in response to information that amount of ink to be ejected by a certain nozzle is more than normal amount, image data is corrected so as to decrease a gradation value indicated by the image data corresponding to the nozzle. In contrast, in response to information that amount of ink to be ejected by a certain nozzle is less than the normal amount, image data is corrected so as to increase a gradation value indicated by the image data corresponding to the nozzle. In this manner, the number of ink dots to be eventually printed is increased or decreased, so that the density of a print image formed based on the image data can become substantially even in each of the nozzles.
In the case where a processing unit for correction by the HS technique is composed of a plurality of print elements, that is, a plurality of print elements whose data is to be corrected based on one and the same correction data, the correction data is generally based on a result obtained by averaging print characteristics of the plurality of print elements. Therefore, an image printed by the plurality of print elements based on the print data after the HS processing per processing unit based on the average correction data comes to have a difference in density according to a difference in print characteristics among the plurality of print elements, if any. This difference in density appears, on an image, as density unevenness in a cycle of a processing unit width. In the meantime, a dither matrix to be used for quantizing the data after the HS processing may induce a cyclic pattern of a density distribution on a print image according to its threshold pattern.
Normally, the above-described density unevenness in the cycle of the HS processing unit width or the above-described cyclic pattern of the density distribution due to the dither matrix is designed on an unrecognizable level. However, if the cycle of an HS processing unit width is synchronous with the cycle of a dither matrix, interference unevenness that the density unevenness in the cycle of the HS processing unit width emphasizes the cyclic pattern of the dither matrix in the synchronous cycle may be recognized. Such interference unevenness degrades a quality of a print image.